Fixed 'findStartNimrod' in koch.nim to return full path

Trying to compile cloned nimrod from master on OS X, when I'd previously installed nimrod from downloaded zip-file, I got the error:
```
Thorr:Nimrod skyfex$ ./koch boot
Traceback (most recent call last)
koch.nim(281)            koch
koch.nim(142)            boot
koch.nim(135)            copyExe
os.nim(748)              copyFile
os.nim(205)              OSError
Error: unhandled exception: No such file or directory [EOS]
```

``findStartNimrod`` searches the PATH for a directory with nimrod, but returns only "nimrod", and not the whole path. I think this commits should fix the issue.